Polyesters and polyamides have excellent fiber properties, however, polyesters, especially polyester fibers are difficult to dye. Polyamides are not as difficult to dye as polyesters, but one or more dyeability additives are commonly incorporated into polyamides in order to selectively increase the affinity of the polyamide fibers for certain type of dyes or the resistance of the fibers to staining with certain type of staining agents.
U.S. Pat. No. 6,312,805 B1 discloses a process for the synthesis of SEHA involving a sulfonation reaction of 4-vinylcyclohexane-1,2-diol (VCHD), where the pH of sulfonation reaction is maintained at 6.3-6.5 by adding sodium hydroxide solution occasionally to the reaction system. The pH control method disclosed in U.S. Pat. No. 6,312,805 may be difficult in larger scale since mixing takes considerable time. As a result, the concentration of solution is localized and the pH is not uniform. When the solution pH fluctuates, more impurities can be formed, reducing product yield and creating a difficult purification process.
Furthermore, the use of sodium hydroxide to control the pH results in sodium sulfate in the final product (SEHA) solution. The sodium sulfate has very low solubility and it crystallizes before SEHA does, thus a filtration step to remove sodium sulfate is necessary. Even after the filtration, sodium sulfate residue in SEHA crystals is still difficult to be removed. SEHA crude product may need to be recrystallized repeatedly in order to reduce the sodium sulfate level to ≦0.1%. It would be desirable to have a process for producing SEHA with increased yield and with less generation of impurities so that purification is easier and the cost of production is low. The present invention simplifies the process described in U.S. Pat. No. 6,312,805 by eliminating one filtration step, which also improves purity of SEHA.
JP Kokai 61-282354 describes a sulfonation reaction with hydrogen sulfite salt at pH 6.5-7.0 by using a base, or an inorganic salt, such as sodium hydroxide and sodium sulfite. Such combination allows the reaction to reach high yield.
In many literature reports, about 0.5-1 equivalent of sodium sulfite is combined with 1 equivalent sodium bisulfate. This introduces a significant amount of sodium sulfite into the solution. The product separation can be difficult and costly. Helberger (Angew. Chem., 73, 69, 1961) reported that at pH 4, the sulfonation reaction of allyl alcohol produces virtually all the sulfonate-sulfinate bisubstituted product. To get a mono substituted sulfonate, one has to carry out the reaction at a pH higher than 4. G. Wenz and T. Holfer carried out sulfonation reaction on cyclodextrin allyl ethers (Carbohydrate Research 322, 153-165, 1999) using KNO3 as the initiator and sodium hydroxide to adjust pH to 7.1 M. S. Kharasuch et al (J. Org. Chem., V.3 175, 1938) used ammonia to buffer the sulfonation reaction of sodium bisulfate with vinyl group.
U.S. Pat. No. 2,793,229 discloses a process in which a buffer system to make the sulfonation reaction most efficient. Dilute mineral acids are used, namely sulfuric acid.
It is desirable to have a process to produce SEHA wherein there is an ability to control the reaction to the direction of forming mono-substitution. The present invention provides such a process, by employing a minimum amount of a buffer system.